As the cost of training pilots in actual vehicles, such as modern commercial and military aircraft, large ships, trucks, etc., has risen, there has been an increased demand for better simulators and trainers in which training can be done at much lower cost. One way of improving simulation fidelity and the training value of such devices, is to include therein a display system which can provide the trainee with realistic visual cues. Ideally, such a visual display should provide an image that appears to be at infinity, over as large a field of view as is visible from the actual vehicle.
One of the best ways of developing an image at infinity is to use a spherical mirror as a collimator. In a well known simulator display system of this genre, a beamsplitter and concave spherical mirror are located in front of a trainee. The beamsplitter is used to fold the optical axis so that an input image source can be located out of the direct field of view of the trainee, at an elevated position optically equivalent to the focal surface of the spherical mirror. The input image is projected from said source down onto the beamsplitter, from which it is reflected toward the concave surface of the spherical mirror. The visual scene is then reflected back from the mirror, through the beamsplitter, to the trainee's eye, which is located on the optical axis of the display, near the center of curvature of the mirror.
Although any one of a number of available image generators can be employed with the above-described infinity image display, frequently a cathode ray tube (CRT), having its faceplate located in the focal surface of the spherical mirror, is used as the image source for the display. This CRT-mirror-beamsplitter combination is widely used in vehicle simulators, especially flight simulators, to provide realistic out-of-the-window scenes.
The primary deficiency of CRT-mirror-beamsplitter display systems is that they provide only a limited vertical field of view. The CRT limits the top of the field of view, and the intersection of the beamsplitter and the mirror limits the bottom field of view. The vertical field of view of such display systems is thus constrained to approximately 31.degree.. Such display systems are disclosed in U.S. Pat. Nos. 3,432,219; 3,659,920; and 4,112,462.
As a result of efforts to increase the field of view of the above-described visual display systems, two mirror systems have been developed. Two mirror systems developed to date include a projection mirror, an eyepiece mirror, two path-folding beamsplitters and a curved input image surface such as a CRT. The projection mirror images the input surface at the focal surface of the eyepiece mirror, which image is reflected by the eyepiece mirror and focused at infinity. The range of vertical and horizontal fields of view is extremely large. Typical two mirror systems may exhibit horizontal and vertical fields of view, for example, of 67.degree. by 65.degree. or 110.degree. by 50.degree.. A plurality of two mirror systems may be disposed in a side-by-side array to form a mosaic system thereby increasing either the vertical field of view or the horizontal field of view or both. Moreover, although a two mirror system normally requires a great amount of space, a fold mirror may be utilized to make the system more compact. Such two mirror display systems are disclosed in U.S. Pat. Nos. 3,549,803; 3,709,581; and 3,785,715.
The major drawback of present two mirror visual display systems is that the systems are light inefficient, i.e., the viewed image is extremely faint relative to the brightness of the input image. Assuming each mirror reflects approximately eighty percent of the light received and each beamsplitter is fifty percent reflective and fifty percent transmissive, the viewed image brightness is only four percent of the input image brightness. Consequently, there is a need for an infinity image visual display system that offers large vertical and horizontal fields of view while being relatively light efficient. Ideally such a system should be adaptable for mosaicing.